In general, image-forming apparatuses such as electrophotographic (including electrostatic recording) copiers, facsimile machines, and laser beam printers form an image by carrying out a series of steps including a charging step of evenly and uniformly charging a surface of a photoconductor; a step of exposing an image to form an electrostatic latent image on the photoconductor; a developing step of causing a toner (developer) to adhere to the electrostatic latent image to form a toner image; a transfer step of transferring the toner image on the photoconductor onto a recording medium such as paper or an overhead projector (OHP) sheet; and a fixing step of fixing the unfixed toner image to the recording medium.
In general, in the fixing step, the unfixed toner image on the recording medium is heated and pressurized to fix the toner image to the recording medium. In the fixing step, a fixing unit (also referred to as “fixing device”) equipped with a roller pair including a fixing roller having built-in heating means such as an electric heater and a pressure roller arranged to oppose the fixing roller is usually used. A recording medium on which an unfixed toner image has been formed is passed through a nip portion formed by bringing the two rollers into contact with each other so as to heat the recording medium under pressure and fix the unfixed toner image to the recording medium. In the fixing unit, the fixing roller and the pressure roller constituting the roller pair are each rotatably supported and rotated synchronously.
In recent years, a fixing technique has been developed in which an unfixed toner image on a recording medium is heated under pressure by substantially directly using a heater installed in a thin-walled tubular or endless-belt-shaped fixing belt arranged to oppose a pressure roller or belt. This fixing technique can significantly shorten the standby time after power is turned on and contributes to weight- and size-reduction of image-forming apparatuses.
Image-forming apparatuses use various types of rollers and belt members, such as charging rollers, charging belts, transfer rollers, and transfer belts, in steps other than the fixing step. Various types of rollers and belt members used in an image-forming apparatus are required to have functions that are suitable for the respective usages (e.g., a charging roller to have chargeability).
Among these functions, one of the common functions or characteristics is the requirement of high toner releasability that prevents a toner from adhering to surfaces and avoids occurrence of toner filming. In particular, a fixing roller or belt comes into close contact with an unfixed toner image on a recording medium under pressure and heating and thus is particularly required to have high toner releasability at its surface. When residual toner remains or toner filming occurs on a surface of a fixing roller or belt, a high-quality image cannot be formed.
In addition, these rollers or belt members are sometimes required to have an adequate degree of elasticity at their surfaces. For example, a fixing roller or belt is often required to have an ability to heat and pressurize an unfixed toner image on a recording medium while deforming around the unfixed toner image. A pressure roller or belt arranged to oppose the fixing roller or belt is also required to have an elastic surface.
Various rollers and belt members of image-forming apparatuses generally include as a main constituent component a columnar, cylindrical, tubular, or endless-belt-shaped base member. In order to impart toner releasability to a surface of the roller or belt member, a fluororesin layer is typically formed as the outermost layer of the base member. In order to impart elasticity to a surface of the roller or belt member, a rubber layer is typically formed on a base member by using a heat-resistant rubber material such as fluororubber and/or silicone rubber.
Thus, a fluororesin-coated roller or belt that includes a fluororesin layer directly on a base member or on a rubber layer disposed on the base member is used as the roller or belt member. A fluororesin layer generally has high non-tackiness, wear resistance, and heat resistance. However, in order to form a fluororesin layer directly on the base member or on a rubber layer on the base member, firing at a high temperature are needed or long hours of heating at a high temperature is needed to fusion-bond the fluororesin layer. Thus, not only has it been difficult to further improve the productivity of producing fluororesin-coated rollers or belts, but there has also been a problem in that the rubber layer underneath becomes thermally deteriorated during formation of the fluororesin layer, thereby decreasing its durability. Various production methods related to fluororesin-coated rollers or belts have been proposed but it has been difficult to sufficiently address the challenges described above.
A method for producing a fluororesin-coated roller or belt is known with which a coating film of a fluororesin coating material is formed on a base member or a rubber layer on a base member and fired at high temperature. A homogeneous fluororesin layer having a smooth flat surface can be formed by firing the fluororesin coating film.
For example, U.S. Pat. No. 3,584,682 (PTL 1) discloses a fixing member that includes a carrier, an adhesive layer disposed on the carrier, the adhesive layer containing a fluororesin and a silicone-modified polyimide, and an outermost layer composed of a fluororesin. Examples disclosed in PTL 1 include an experimental example in which a fluororesin layer is formed by electrostatically coating an adhesive layer with a powder coating material of a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) and a curing treatment and film-forming treatment are performed under firing conditions of 375° C. and 30 minutes.
Japanese Unexamined Patent Application Publication No. 10-198201 (PTL 2) discloses a fixing belt that has a layered structure including a thin-walled tubular base member, a heat resistant elastomer layer on the outer peripheral surface of the base member, and a fluororesin layer on the heat resistant elastomer layer. Examples of PTL 2 include an experimental example in which a silicone rubber layer is formed on an outer peripheral surface of a polyimide tube by press forming and a fluororesin coating material is applied to the silicone rubber layer and sintered to form a fluororesin layer.
According to the firing methods disclosed in PTL 1 and PTL 2, firing must be conducted for a long time in a dry heat atmosphere at high temperature equal to or higher than the melting point of the fluororesin in order to form a fluororesin layer. In the case where a rubber layer is present beneath the fluororesin layer, thermal deterioration of the rubber layer will occur during firing of the fluororesin layer since the heat resistant temperature of the rubber layer is lower than the firing temperature of the fluororesin layer. In particular, thermal deterioration of the rubber layer surface easily occurs at the interface between the fluororesin layer and the rubber layer. As a result, the interlayer adhesiveness may be degraded and interlayer separation may occur. The durability of the fluororesin-coated roller or belt is decreased as a result.
Another known method for producing a fluororesin-coated roller or belt is a method including forming a fluororesin layer on an inner surface of a cylindrical die, inserting a roller-shaped base member at the axial center of the cylindrical die, injecting a rubber material into a gap between the fluororesin layer and the roller-shaped base member, and vulcanizing the rubber material. After vulcanization, a fluororesin-coated roller having a “fluororesin layer/rubber layer/base member” layered structure is removed from the cylindrical die.
For example, Japanese Unexamined Patent Application Publication No. 11-336742 (PTL 3) and Japanese Unexamined Patent Application Publication No. 2001-295830 (PTL 4) each disclose a method for producing a fluororesin-coated roller or belt, the method including step 1 of applying a fluororesin coating material or a fluororesin powder coating material onto an inner surface of a cylindrical die to form a coating film and firing the coating film to form a fluororesin film; step 2 of inserting a roller-shaped base member at the axial center of the cylindrical die; step 3 of injecting a rubber material into a gap between the fluororesin coating film and the roller-shaped base member and vulcanizing the rubber material; and step 4 of removing the cylindrical die.
According to the casting method disclosed in PTL 3 and PTL 4, firing of the fluororesin layer that forms the outermost layer is performed in the first step and vulcanization of the rubber layer is performed thereafter. Thus, thermal deterioration of the rubber layer caused by firing of the fluororesin layer can be prevented. However, according to the casting method disclosed in PTL 3 and PTL 4, a high-precision cylindrical die is needed and it is difficult to accurately align the axial center of the roller-shaped base member with the axial center of the cylindrical die. Moreover, according to the casting method, problems such as it being difficult to form a thin rubber layer, the thickness of the rubber layer becoming uneven, and there being a possibility of the fluororesin film breaking during removal of the die are likely to occur.
Another known method for producing a fluororesin-coated roller or belt is a method including a step of inserting a rubber-coated roller base member into a fluororesin tube having no thermal shrinkability while radially expanding an opening portion of the fluororesin tube.
For example, Japanese Unexamined Patent Application Publication No. 2004-276290 (PTL 5) and Japanese Unexamined Patent Application Publication No. 2008-257098 (PTL 6) each disclose a method for producing a fluororesin-coated roller or belt, the method including applying a low-viscosity adhesive onto an outer peripheral surface of a rubber-coated roller base member and covering the rubber-coated roller base member with a fluororesin tube having an inner diameter smaller than the outer diameter of the rubber-coated roller base member while radially expanding one end of the fluororesin tube. After the covering step, there is a heat-curing step for the adhesive layer.
The fluororesin tube covering method disclosed in PTL 5 and PTL 6 has the following problems: a complex, expensive machine is needed to expand the inner diameter of the fluororesin tube; a low-viscosity adhesive must be applied onto a rubber layer surface of the rubber-coated roller base member so as to impart lubricity that allows the base member to be covered with the fluororesin tube; the thickness of the adhesive layer tends to vary; bubbles are easily generated between the fluororesin tube and the layer underneath due to entrapment of air; heating at high temperature must be conducted for a long time to cure the adhesive; and durability may be insufficient due to failure to fusion-bond the fluororesin tube to the rubber layer.
Yet another known method for producing a fluororesin-coated roller or belt is a method including steps of covering a base member or a rubber layer on the base member with a thermally shrinkable fluororesin tube, performing heating to conduct thermal shrinkage, and further conducting heating at high temperature for a long time to carry out fusion bonding.
For example, Japanese Unexamined Patent Application Publication No. 64-1534 (PTL 7) discloses a method for producing a fluororesin-coated columnar article, the method including covering a primer-treated columnar article with a thermally shrinkable PFA tube, shrinking and fixing the tube in place by heating at a temperature in a range of 80° C. to 250° C., and firing the tube at a temperature in a range of 330° C. to 400° C. to conduct fusion bonding.
U.S. Pat. No. 3,112,335 (PTL 8) discloses a method for producing a tubular composite film, the method including covering an outer peripheral surface of a polyimide tube with a thermally shrinkable PFA tube and conducting firing by heating at a temperature of 280° C. to 400° C. for 30 to 60 minutes (in Example 1, for 40 minutes at 350° C.) to fusion-bond the thermally shrinkable PFA tube.
International publication No. 2008/126915 (PTL 9) discloses a method for producing a fixing roller or belt, the method including forming an elastic layer on a base member such as a stainless steel cylinder, covering the elastic layer with a thermally shrinkable PFA tube, thermally shrinking the tube in a 290° C. to 300° C. atmosphere to bring the tube into close contact with the elastic layer, and conducting heating at a temperature equal to or higher than the melting point of PFA to achieve fusion bonding.
The methods of covering a base member with a thermally shrinkable fluororesin tubes disclosed in PTL 7 to PTL 9 have the following problems: bubbles are easily generated between the base member and the thermally shrinkable fluororesin tube or between the rubber layer and the thermally shrinkable fluororesin tube; slack and wrinkles are easily generated in the thermally shrinkable fluororesin tube during thermal shrinking; a heating treatment in a dry heat atmosphere must be conducted for a long time to fusion-bond the thermally shrinkable fluororesin tube; and a rubber layer underneath is susceptible to thermal deterioration during fusion-bonding of the thermally shrinkable fluororesin tube.
Some of PTL 7 to PTL 9 state that the thermally shrinkable fluororesin tube can be fusion-bonded onto the outer peripheral surface of a base member or a rubber layer by conducting heating for a relatively short time. However, in practice, in order to fusion-bond a thermally shrinkable fluororesin tube, heating must be performed for a long time in a dry heat atmosphere at high temperature. If the heating temperature is excessively low or the heating time is excessively short, fusion-bonding of the thermally shrinkable fluororesin tube onto the outer peripheral surface of the base member or the rubber layer becomes insufficient and the durability of the fluororesin-coated roller or belt is impaired.